Network Working Group M. Tuexen
Internet-Draft I. Ruengeler
Updates: 4960 (if approved) Muenster Univ. of Appl. Sciences
Intended status: Standards Track R. Stewart
Expires: March 01, 2014 Adara Networks
August 28, 2013
SACK-IMMEDIATELY Extension for the Stream Control Transmission Protocol
draft-ietf-tsvwg-sctp-sack-immediately-04.txt
Abstract
This document updates RFC 4960 by defining a method for the sender of
a DATA chunk to indicate that the corresponding SACK chunk should be
sent back immediately and not be delayed. It is done by specifying a
bit in the DATA chunk header, called the I-bit, which can get set
either by the SCTP implementation or by the application using an SCTP
stack. Since unknown flags in chunk headers are ignored by SCTP
implementations, this extension does not introduce any
interoperability problems.
Status of This Memo
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This Internet-Draft will expire on March 01, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The I-bit in the DATA Chunk Header . . . . . . . . . . . . . 3
4. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Triggering at the Application Level . . . . . . . . . . . 4
4.2. Triggering at the SCTP Level . . . . . . . . . . . . . . 4
5. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Sender Side Considerations . . . . . . . . . . . . . . . 5
5.2. Receiver Side Considerations . . . . . . . . . . . . . . 5
6. Interoperability Considerations . . . . . . . . . . . . . . . 5
7. Socket API Considerations . . . . . . . . . . . . . . . . . . 5
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
9. Security Considerations . . . . . . . . . . . . . . . . . . . 7
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
11.1. Normative References . . . . . . . . . . . . . . . . . . 7
11.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
According to [RFC4960] the receiver of a DATA chunk should use
delayed SACKs. This delaying is completely controlled by the
receiver of the DATA chunk and remains the default behavior.
In specific situations the delaying of SACKs results in reduced
performance of the protocol:
1. If such a situation can be detected by the receiver, the
corresponding SACK can be sent immediately. For example,
[RFC4960] recommends the immediate sending if the receiver has
detected message loss or message duplication.
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2. However, if the situation can only be detected by the sender of
the DATA chunk, [RFC4960] provides no method of avoiding a delay
in sending the SACK. Examples of these situations include ones
which require interaction with the application (e.g. applications
using the SCTP_SENDER_DRY_EVENT, see Section 4.1) and ones which
can be detected by the SCTP stack itself (e.g. closing the
association, hitting window limits or resetting streams, see
Section 4.2).
To overcome the limitation described in the second case, this
document describes a simple extension of the SCTP DATA chunk by
defining a new flag, the I-bit. The sender of a DATA chunk indicates
by setting this bit that the corresponding SACK chunk should not be
delayed.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. The I-bit in the DATA Chunk Header
The following Figure 1 shows the extended DATA chunk.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0 | Res |I|U|B|E| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream Identifier | Stream Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Protocol Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ User Data /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Extended DATA chunk format
The only difference between the DATA chunk in Figure 1 and the DATA
chunk defined in [RFC4960] is the addition of the I-bit in the flags
field of the DATA chunk header.
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This bit was Reserved in [RFC4960]. [RFC4960] specified that this
bit should be set to 0 by the sender and ignored by the receiver.
4. Use Cases
The setting of the I-bit can either be triggered by the application
using SCTP or by the SCTP stack itself. The following two
subsections provide a non-exhaustive list of examples.
4.1. Triggering at the Application Level
One example of a situation in which it may be desirable for an
application to trigger setting of the I-bit involves the
SCTP_SENDER_DRY_EVENT in the SCTP socket API [RFC6458]. Upper layers
of SCTP using the socket API as defined in [RFC6458] may subscribe to
the SCTP_SENDER_DRY_EVENT for getting a notification as soon as no
user data is outstanding anymore. To avoid an unnecessary delay
while waiting for such an event, the application can request the
setting of the I-Bit when sending the last user message before
waiting for the event. This results in setting the I-bit of the last
DATA chunk corresponding to the user message and is possible using
the extension of the socket API described in Section 7.
4.2. Triggering at the SCTP Level
There are also situations in which the SCTP implementation can set
the I-bit without interacting with the upper layer.
If the association is in the SHUTDOWN-PENDING state, setting the
I-bit reduces the number of simultaneous associations for a busy
server handling short living associations.
Another case is where the sending of a DATA chunk fills the
congestion or receiver window. Setting the I-bit in these cases
improves the throughput of the transfer.
If an SCTP association supports the SCTP Stream Reconfiguration
extension defined in [RFC6525], the performance can be improved by
setting the I-bit when there are pending reconfiguration requests
that require that there be no outstanding DATA chunks.
5. Procedures
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5.1. Sender Side Considerations
Whenever the sender of a DATA chunk can benefit from the
corresponding SACK chunk being sent back without delay, the sender
MAY set the I-bit in the DATA chunk header. Please note that it is
irrelevant to the receiver why the sender has set the I-bit.
Reasons for setting the I-bit include, but are not limited to, the
following (see Section 4 for the benefits):
o The application requests to set the I-bit of the last DATA chunk
of a user message when providing the user message to the SCTP
implementation (see Section 7).
o The sender is in the SHUTDOWN-PENDING state.
o The sending of a DATA chunk fills the congestion or receiver
window.
o The sending of an Outgoing SSN Reset Request Parameter or an SSN/
TSN Reset Request Parameter is pending, if the association
supports the Stream Reconfiguration extension defined in
[RFC6525].
5.2. Receiver Side Considerations
On reception of an SCTP packet containing a DATA chunk with the I-bit
set, the receiver SHOULD NOT delay the sending of the corresponding
SACK chunk, i.e., the receiver SHOULD immediately respond with the
corresponding SACK chunk.
6. Interoperability Considerations
According to [RFC4960] the receiver of a DATA chunk with the I-bit
set should ignore this bit when it does not support the extension
described in this document. Since the sender of the DATA chunk is
able to handle this case, there is no requirement for negotiating the
support of the feature described in this document.
7. Socket API Considerations
This section describes how the socket API defined in [RFC6458] is
extended to provide a way for the application to set the I-bit.
Please note that this section is informational only.
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A socket API implementation based on [RFC6458] needs to be extended
to allow the application to set the I-bit of the last DATA chunk when
sending each user message.
This can be done by setting a flag called SCTP_SACK_IMMEDIATELY in
the snd_flags field of the struct sctp_sndinfo structure when using
sctp_sendv() or sendmsg(). If the deprecated struct sctp_sndrcvinfo
structure is used instead when calling sctp_send(), sctp_sendx(), or
sendmsg(), the SCTP_SACK_IMMEDIATELY flag can be set in the
sinfo_flags field. When using the deprecated function sctp_sendmsg()
the SCTP_SACK_IMMEDIATELY flag can be in the flags parameter.
8. IANA Considerations
[NOTE to RFC-Editor:
"RFCXXXX" is to be replaced by the RFC number you assign this
document.
]
Following the chunk flag registration procedure defined in [RFC6096],
IANA should register a new bit, the I-bit, for the DATA chunk. The
suggested value is 0x08 and the reference should be RFCXXXX.
This requires an update of the "DATA Chunk Flags" registry for SCTP:
DATA Chunk Flags
+------------------+-----------------+-----------+
| Chunk Flag Value | Chunk Flag Name | Reference |
+------------------+-----------------+-----------+
| 0x01 | E bit | [RFC4960] |
| 0x02 | B bit | [RFC4960] |
| 0x04 | U bit | [RFC4960] |
| 0x08 | I Bit | [RFCXXXX] |
| 0x10 | Unassigned | |
| 0x20 | Unassigned | |
| 0x40 | Unassigned | |
| 0x80 | Unassigned | |
+------------------+-----------------+-----------+
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9. Security Considerations
See [RFC4960] for general security considerations for SCTP. In
addition, a malicious sender can force its peer to send packets
containing a SACK chunk for each received packet containing DATA
chunks instead of every other. This could impact the network,
resulting in more packets sent on the network, or the peer because
the generating and sending of the packets has some processing cost.
However, the additional packets can only contain the most simplest
SACK chunk (no gap reports, no duplicate TSNs), since in case of
packet drop or reordering in the network a SACK chunk would be sent
immediately anyway. Therefore this does neither introduce a
significant additional processing cost on the receiver side. This
does not result in more traffic in the network than a receiver that
sends a SACK for every packet, which is already permitted.
10. Acknowledgments
The authors wish to thank Mark Allmann, Brian Bidulock, David Black,
Anna Brunstrom, Gorry Fairhurst, Janardhan Iyengar, Kacheong Poon,
and Michael Welzl for their invaluable comments.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC
4960, September 2007.
[RFC6096] Tuexen, M. and R. Stewart, "Stream Control Transmission
Protocol (SCTP) Chunk Flags Registration", RFC 6096,
January 2011.
11.2. Informative References
[RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
Yasevich, "Sockets API Extensions for the Stream Control
Transmission Protocol (SCTP)", RFC 6458, December 2011.
[RFC6525] Stewart, R., Tuexen, M., and P. Lei, "Stream Control
Transmission Protocol (SCTP) Stream Reconfiguration", RFC
6525, February 2012.
Authors' Addresses
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Michael Tuexen
Muenster University of Applied Sciences
Stegerwaldstr. 39
48565 Steinfurt
DE
Email: tuexen@fh-muenster.de
Irene Ruengeler
Muenster University of Applied Sciences
Stegerwaldstr. 39
48565 Steinfurt
DE
Email: i.ruengeler@fh-muenster.de
Randall R. Stewart
Adara Networks
Chapin, SC 29036
US
Email: randall@lakerest.net
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